A fiber optic cable may include one or more optical fibers capable of transmitting audio, video or other information. Examples of optical fibers are disclosed in U.S. Pat. Nos. 5,561,730 and 5,457,762. Fiber optic cables are laid over long distances and require optical connectors or ferrules to link discrete segments of the optical fibers. As used herein, the term “ferrule” refers to a plug assembly or a structure that receives a terminal end of an optical fiber or optical fiber ribbon and then abuts against an opposing ferrule to align corresponding optical fiber or ribbon for transmission of an optical signal or signals.
An example of an optical ferrule is disclosed in U.S. Pat. No. 5,214,730 to Nagasawa et al. FIG. 1 illustrates an optical ferrule similar to that depicted in Nagasawa, and shows multi-fiber ferrules 3 and 3′ connected to optical fiber ribbons 1 and 1′, respectively. Ribbon 1 comprises multiple optical fibers 2 to be aligned with corresponding optical fibers 2′ (not shown) from ribbon 1′. Ferrule 3 defines a plurality of optical fiber bores adapted to receive fibers 2 and two guide pin bores 4 adapted to receive guide pins 6. Guide pins 6 align ferrule 3 with ferrule 3′, when the two ferrules are connected to each other to align optical fibers 2 and 2′ to optimize optical transmission.
During a typical molding process to produce ferrules 3, bore forming pins are inserted through the mold cavity to create the guide pin bores and the optical fiber bores in the ferrules. Molten plastic is then injected into the mold cavity, and after the plastic solidifies sufficiently the pins are withdrawn to form the bores in the ferrules to receive the optical fibers and guide pins. Prior to connecting to ferrule 3, optical ribbon 1 is stripped of its outer matrix coating and its buffer layer to expose fibers 2. The individual fibers 2 are inserted into the fiber bores on ferrule 3. Various well-known techniques are used to permanently affix fibers 2 to ferrule 3. End faces 5 and 5′ of ferrules 3 and 3′ are then polished along with the exposed ends of fibers 2. A pair of guide pins 6 is then inserted into guide holes 4 to connect and align the ferrules. A spring clip (not shown) may be used to clamp the two ferrules together.
There is a premium placed on the precise alignment of opposing optical fibers at a connection to minimize signal losses, which diminishes the quality of the optical transmission through the connection. The precision of aligning opposing optical fibers is more sensitive with multi-fiber ferrules due to the presence of multiple optical fibers and to each fiber's location relative to each other and relative to the guide pins within the ferrules. Additionally, when an optical fiber is a single-mode fiber, i.e., the optical signal is transmitted through only a small portion of the fiber, the alignment needs to be even more precise.
A conventional ferrule molding method uses a series of V-shaped open grooves machined into a block of the mold cavity to retain the bore forming pins inserted into the mold cavity. FIG. 2 shows a cross-sectional view of this conventional molding method, where fiber bore forming pins 7 and guide pin bore forming pins 8 are shown disposed in V-shaped grooves 9. The disadvantages of this or similar open groove constructions include a tendency of the pins 7 and 8 to float within the V-shaped grooves in the direction of arrow A during the molding process. This float contributes to imprecise alignment of the bores formed in the molded ferrule. Additionally, after repeated uses of a mold cavity with this groove construction, flash begins to build up in areas indicated by B. This flash build up requires frequent cleaning of the grooves. Also, as can be seen, pins 7 contact the V-shaped grooves only along two lines of contact and thus all the friction forces of the repeated insertion and removal of the pins are imparted along these two lines of contact, thereby causing uneven wear along the sides of the V-shaped groove. This causes the alignment of the pins to become progressively more imprecise.
The drawbacks of the molding process with the V-shaped grooves have been addressed by the “small hole technology” disclosed by U.S. Pat. No. 5,786,002 to Dean et al. As shown in FIG. 3, Dean et al. discloses a guide block assembly comprising a plurality of fiber bore blocks 12, at least two guide pin bore blocks 14 and a plurality of spacer blocks 16 arranged in any desirable configuration in a mold cavity. Each fiber bore block 12 defines a small hole or bore 18 adapted to receive during the molding process a pin having the diameter of an optical fiber, and each guide pin bore blocks 14 defines a bore 19 adapted to receive a pin having a diameter of a guide pin. Molten plastic is injected into the mold cavity and the pins are thereafter withdrawn from the holes and the mold cavity to form receptacles in the ferrules to receive optical fibers 2 or guide pins 6. The use of bores more precisely retains the pins during the molding process than the use of V-shaped open grooves. Dean et al. resolves the known drawbacks from the V-shaped open groove molding technique, and provides the additional benefits of establishing precise spatial relationship among the modular blocks, by machining the surfaces of the adjoining blocks.
Dean et al., however, requires the fabrication of multiple blocks, which increases the costs and may become less economical when used to fabricate ferrules for a small number of optical fibers. Hence, there remains a need in the art for a molding apparatus that has the advantages realized in the Dean et al. '002 patent, but requires fewer components and is more economical to produce.